Unlike polymers obtainable by ionic polymerization or polycondensation, functional group-containing vinyl polymers, in particular functional group-terminated vinyl polymers, obtainable by radical polymerization have scarcely been put to practical use. Among vinyl polymers, (meth)acrylic polymers have such characteristics as high weatherability and transparency that polyether polymers, hydrocarbon polymers or polyester polymers cannot have, and (meth)acrylic polymers having an alkenyl or crosslinking silyl group(s) on a side chain(s) are currently used in highly weather-resistant coating compositions and the like. On the other hand, it is not easy to control the polymerization of acrylic polymers because of side reactions, and it is very difficult to introduce a functional group into such polymers at one or both ends.
If vinyl polymers having an alkenyl group at a molecular chain terminus or termini can be obtained by a simple and easy method, it becomes possible to obtain cured products superior in cured product properties to cured products from vinyl polymers having a crosslinking group(s) on a side chain(s). Therefore, a number of researchers have so far made investigations concerning the method for the production thereof. However, it is not easy to produce them on a commercial scale. In Japanese Kokai Publication Hei-01-247403 and Japanese Kokai Publication Hei-05-255415, for instance, there is disclosed a method of synthesizing alkenyl-terminated (meth)acrylic polymers which uses an alkenyl group-containing disulfide as a chain transfer agent.
Japanese Kokai Publication Hei-05-262808 discloses a method of synthesizing alkenyl-terminated (meth)acrylic polymers which comprises synthesizing a vinyl polymer having a hydroxyl group at each end using a hydroxyl group-containing disulfide and further utilizing the reactivity of each hydroxyl group.
Japanese Kokai Publication Hei-06-211922 discloses a method of synthesizing silyl group-terminated (meth)acrylic polymers which comprises synthesizing a vinyl polymer having a hydroxyl group at each end using a hydroxyl group-containing polysulfide and utilizing the reactivity of each hydroxyl group.
These methods can hardly ensure that a desired functional group will be introduced at each of both ends. Hence, cured products having satisfactory characteristics cannot be obtained. For introducing a functional group at each of both ends without fail, a chain transfer agent must be used in large amounts, and this is a problem from the production process viewpoint. In addition, these methods use an ordinary method of radical polymerization, so that it is difficult to control the molecular weight and molecular weight distribution (ratio of weight average molecular weight to number average molecular weight) of the product polymer.
In view of such a state of the art, the present inventors have so far made a number of inventions relating to various crosslinkable functional group-terminated vinyl polymers, methods of producing the same, curable compositions comprising the same, and uses thereof (cf. Japanese Kokai Publication Hei-11-080249, Japanese Kokai Publication Hei-11-080250, Japanese Kokai Publication Hei-11-005815, Japanese Kokai Publication Hei-11-116617, Japanese Kokai Publication Hei-11-116606, Japanese Kokai Publication Hei-11-080571, Japanese Kokai Publication Hei-11-080570, Japanese Kokai Publication Hei-11-130931, Japanese Kokai Publication Hei-11-100433, Japanese Kokai Publication Hei-11-116763, Japanese Kokai Publication Hei-09-272714, and Japanese Kokai Publication Hei-09-272715, among others).
For example, vinyl polymers having a silicon-containing group comprising hydroxyl or hydrolyzable group(s) bound to a silicon atom and capable of crosslinking under siloxane bond formation by moisture or the like even at room temperature (hereinafter, such silicon-containing group is also referred to as “crosslinkable silyl group”) or cured products obtainable from compositions comprising the same are excellent in heat resistance and weatherability and can be used in various fields of application which include, but are not limited to, sealing materials, for example sealing materials such as elastic sealing materials for building and construction and sealing materials for laminated glass, electric and electronic part materials such as solar cell back sealers, electric insulating materials such as wire/cable insulating sheath, pressure sensitive adhesive materials, adhesives, elastic adhesives, paints, powder paints, coating compositions, foamed bodies, potting materials for electric and electronic use, films, gaskets, casting materials, various molding materials, and rustproof and waterproof sealants for end faces (cut sections) of net glass or laminated glass.
In addition, said vinyl polymer or the cured product thereof maybe used to seal an automobile part, an electric part, various kinds of machine parts and the like. Conventionally, as methods for joining and sealing an automobile parts, an electric part or various kinds of machine parts in assembling line, it has been employed a press-joining method which comprises intervening a mold gasket between sealing surfaces, or a sealing method which comprises intervening a liquid gasket between sealing surfaces. As for the sealing method which comprises intervening a liquid gasket between sealing surfaces of the parts, Formed-In-Place-Gasket (=FIPG) method, which comprises sealing the parts with automatically coating a liquid sealant on assembling line with an aid of a robot or the like, has been mainly employed in view of high productivity, low cost and reliability of sealing performance. Materials mainly used for FIPG include room temperature-curable silicone materials capable of curing by reaction with atmospheric moisture at room temperature (silicone RTV) and anaerobic gaskets based on urethane acrylate, which is capable of curing in a short time by nipping said urethane acrylate with flange surfaces in order to shut off from oxygen and to allow the gasket to contact with a metal surface. Among them, the silicone RTV is used most frequently in view of good heat resistance, good workability, good fitness against flange surfaces to be coated, or the like properties.
However, there is a problem about oil resistance in FIPG using a silicone RTV, since high performance various oils such as engine oils have been developed recently and damages of silicone rubber caused by such oils becomes serious. Up to now, various modifications have been tried to overcome the problem concerning oil resistance. As such methods, there may be mentioned a method which comprises incorporating magnesium oxide into silicone RTV (Japanese Kokai Publication Sho-57-76055), a method which comprises incorporating zinc carbonate, thiazol, thiuram and dicarbamic acid salt into silicone RTV (Japanese Kokai Publication Sho-59-80463), a method which comprises incorporating an alkali metal salt of a weak acid with acid dissociation constant of 2.0 to 12.0 into silicone RTV (Japanese Kokai Publication Sho-62-11768), a method which comprises incorporating a silane compound having one vinyl group and one hydroxyl group in one molecule (Japanese Kokai Publication Hei-1-14272), and a method which comprises incorporating iminoxysilane and a basic zinc carbonate having zinc hydroxide content of from 5 to 50% by weight, into silicone RTV (Japanese Kokai Publication Hei-3-203960). However, damages of silicone becomes serious when some oils belonging to SJ grade engine oils, which are recent high-performance engine oils, transmission oils for automatic cars, or gear oils, are used. Therefore, it becomes impossible to solve the problem by the methods described above.
On the other hand, as materials for FIPG which are excellent in oil resistance, there may be mentioned anaerobic resins whose main component is urethane (meth)acrylate resin, described in Japanese Kokai Publication Sho-58-187481, Japanese Kokai Publication Sho-64-112, Japanese Kokoku Publication Hei-3-32593 and Japanese Kokai Publication Sho-61-2719. However, said resins have a problem concerning long-term heat resistance since they have an ether bond and/or an ester bond in the main chain thereof. In addition, a cured product of the urethane (meth)acrylate is inferior in flexibility to that of a silicone resin, and thus there are some defects. Therefore, the present invention has an object to provide an oil resistant liquid gasket with maintained heat resistance, and a material for the gasket.